1. Field of the Invention
The present invention relates to an improved method and apparatus for making a molded product in which a molten resin is transported by a resin injection device into a mold cavity, comprised by a first mold having a molten resin passage and a second mold having the product cavity.
2. Background of the Invention
A synthetic resin forming apparatus in general use to produce resin products operates by molten resin by means of a resin injection device into a mold cavity, comprising a first mold and a second mold, through a molten resin passage disposed on the first mold. In such a resin forming apparatus, there is a need to reduce the formation of unwanted debris formed by the hardened residual molten resin left in the molten resin passage. For solving such a problem, the present inventors had created a mold design in which the molten resin passage is extended to the second mold, and, at the same time, a moving device is provided as a unit with the second mold such that a rod-shaped member operates in the molten resin passages to move back and forth in the molten resin passages.
The design of this type of resin molding apparatus is such that, in the retracted condition of the rod member of the moving device, the rod member is disposed in the molten resin passage of the second mold, and the molding cavity is communicated with the resin injection device. In the forward condition of the rod member, the rod member is disposed such that it projects into the molten resin passage of the first mold. Accordingly, when performing an injection operation, the rod member is retracted so that the resin can be injected into the mold cavity by the resin injection device, and after the injection operation has been completed, the rod member is thrust forth to return any residual resin left in the molten resin passage towards the resin injection device.
However, in a resin forming apparatus of this design, because the rod member is disposed integrally with the transfer device on the second mold side, when the first mold is separated from the second mold, after the completion of the injection operation, the rod member is disengaged from the first mold. This will expose the molten resin passage to ambient air, and molten resin flows out of the open exposure.
Further in the above apparatus, because the molten resin passage of the first mold becomes open as a result of the first mold being separated from the second mold, there is a chance of accumulating unwanted air in the molten resin passage. If the next molding operation is performed in this condition, the air is sometimes pumped into the mold cavity before the resin is charged, and if this happens, the air becomes compressed in the mold cavity by the molten resin thus generating high temperature in the molded product. The causes a phenomenon called burning in the product.